The goal of the research proposed in this request for an ADAMHA RSDA is to improve our understanding of hormonal influences on mammalian social behaviors affecting reproduction and communication. This proposal focuses on behavioral and neural effects of testosterone (T), the primary hormone of the testes. T profoundly affects the development and adult expression of masculine social behaviors in all vertebrates, including humans. Because the preoptic area (POA) of the brain plays an important role in mediating T's effects on sexual and other forms of social motivation in adult males, the first part of the proposal emphasizes T's action on the POA. It capitalizes on the recent discovery by this laboratory of sex differences in the anatomy and histochemistry of the gerbil POA that are influenced by T in adulthood. This is the first time cellular markers of T's effects on the adult brain have been identified in mammals and the first time such markers have ever been identified in brain areas related to motivation. Thus they provide a unique opportunity for studying T's effects on behavior in relation to its effects on specific brain cells. The proposed research would define more precisely the nature if T's effects in these neurons and would study their relationship to two male social behaviors, sexual behavior and scent marking, controlled by T action on the POA in this species. The second part of the proposal concerns T's effects on sexual development. It pursues a recent discovery by this laboratory that the left and right sides of the POA and mediobasal hypothalamus (MBH) are not equally sensitive to T's masculinizing and defeminizing effects on sexual behavior and pituitary function. Using sexual differentiation in rats as a model system, the proposed research tests the hypothesis that accelerated development on the left side of the brain causes the left and right sides of the MBH-POA to pass through the critical periods of masculinization and defeminization at different times. This implies that one side of the brain may exert more control than the other over male sexual behavior, female sexual behavior, and pituitary function and that the side exerting this control can vary with the age at which the individual is exposed to steroid hormones.